The subject matter disclosed herein generally relates to aluminum electroplating process control.
Aluminum (Al) offers a number of beneficial properties that make it useful for a variety of applications, including as a coating for corrosion protection or other purposes. Unlike some other metal coatings, aluminum coatings cannot be deposited using an aqueous electrolyte electroplating process, as the electrochemical conditions that would be necessary to cause the deposition of aluminum also cause water to decompose, forming hydrogen and oxygen before the metals can be deposited. An electroplating process for aluminum was developed by Ziegler and Lehmkuhl in the 1950's, and was later commercially developed and came to be known as the Sigal process. This process utilizes a highly air- and water-sensitive mixture of organoaluminum (triethlyaluminium) compounds, aromatic solvents, and other additives such as alkali halides or hydrides and quaternary onium salts. The pyrophoric triethlyaluminium compound can spontaneously decompose in air by reaction with atmospheric oxygen and moisture, resulting in flame formation and the concomitant handling and safety issues. Ion vapor deposition (IVD) can also be used to deposit aluminum layers; however, as a physical vapor deposition process it is subject to limitations in deposition rate, which can lead to limitations on layer thickness, or excessive cost and difficulty of use. Electroplating Al from ionic liquids (IL's), on the other hand, is considered an environmentally friendly alternative. Historically, the Lewis acidic 1-ethyl-3-methylimidazolium chloride [EMIM][Cl]—AlCl3 systems have been favored since the genesis of Al plating in ILs attributed to Osteryoung, Wilkes and Hussey two decades ago. This process has the potential to offer the benefits of an electroplating process without the handling and safety issues of the Sigal process. This process is, however, highly susceptible to moisture in the operating environment, and even water levels as low as 0.1% wt. of the electrolyte have been shown to produce undesirable aluminum oxychloride side products according to the reaction:Al2Cl7−+H2O→AlOCl+2HCl+AlCl4−Such a low threshold for water content can be difficult to maintain, even when starting materials are dry, as the mere act of accessing the electroplating bath to insert or remove workpiece substrates can result in the accumulation of undesirable levels of moisture in the system. During electroplating, water can enter the IL electroplating bath via vapor/liquid phase boundaries or liquid/liquid phase boundaries if a second liquid phase is disposed on top of the plating solution as a barrier layer. Although numerous water stable and hydrophobic ionic liquids have been discovered and made commercially available, water absorption appears to be inevitable. For instance, a substantial amount of water (ca. 2000 ppm or 0.2% wt.) can be present even for the ILs known to be most hydrophobic, i.e. 1-hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate (HEM-FAP). Thus, minimizing the amount of water in the electroplating bath and operating environment seems to be necessary to commercialize aluminum plating in ILs.
Each of the above-described and other aluminum coating techniques have their own unique set of advantages and disadvantages, such that new and different alternatives are always well received that might be more appropriate for or function better in certain environments or are less costly or more effective.